As various different types of communication systems have arisen for radiotelephones, it has become beneficial to provide portable and mobile radiotelephone stations that are interoperable between these various communication systems. As a first step, dual-mode phones have been developed that can operate between two radiotelephone systems. For example, the Global System for Mobile (GSM) communication system and the Wideband Code Division Multiple Access (WCDMA) communication system are intended to work together in the same mobile terminal equipment operated under a Universal Mobile Telecommunications System (UMTS) environment, wherein a mobile radiotelephone is required to scan for these two alternate radio access technologies (RAT) and all possible operating frequencies for each. In particular, a UMTS radiotelephone is required to scan for these radio access technologies and frequencies when looking for its home Public Land Mobile Network (PLMN), to determine the availability of its home PLMN (HPLMN). In particular, the 3GPP specifications allow for a mobile station to perform (background) scans for PLMNs other than the one on which it has currently obtained service.
Currently, GSM and UMTS cellular radiotelephones are required to perform a periodic search for their home PLMN and higher priority PLMNs whenever the radiotelephone is camped on a Visited PLMN (VPLMN) and in its home country. When outside their home country, the mobile stations are required to perform a search for higher priority PLMNs, but not their home PLMN. This search is required to be performed periodically at a rate which is specified on the Subscriber Identity Module (SIM) or Universal SIM card and is a multiple of six minutes (with the fastest rate being once every six minutes). This search consumes battery power to perform because the radiotelephone must measure power on all frequencies of all bands which it supports and then sync to each frequency on which there is appreciable energy and read the PLMN identification of the cell. Note that for UMTS radiotelephones which support both GSM and WCDMA Radio Access Technologies (RATs), the radiotelephone is currently required by 3GPP specifications to perform the search in both RATs (i.e. the phone must perform the search for the HPLMN in all radio access technologies of which it is capable). This is true even if the HPLMN network has cells of only one RAT. Therefore, the requirement to search for all possible RATs and all possible frequencies wastes significant battery power and is unnecessary.
Further, when camped on a particular Visited PLMN (VPLMN) cell and searching for the HPLMN or higher priority PLMNs, mobile stations can now see energy on (and synchronize to and read the PLMN identification of) the same frequencies over and over again, each time the mobile unit does another search. This occurs even though these frequencies were already found in previous searches to not be the home PLMN or higher priority PLMNs.
Therefore, the need exists for a method to allow a mobile unit to search for the home PLMN and higher priority PLMNs in only those radio access technologies and frequencies that have not already been discounted (i.e. determined to be not of the HPLMN or higher priority PLMNs) during previous scans performed in the same geographic area. It would also be of benefit to provide this performance improvement with little or no additional cost.